1. Field of the Invention
The present invention relates to photolithography steps in manufacturing semiconductor elements, particularly to negative resist materials suitable for lithography employing for exposure lignt having a wavelength of 220 nm or less, to a pattern formation method, and to a method of manufacturing semiconductor devices using the pattern formation method.
2.. Background Art
In the field of manufacture of electronic devices which require micro-processing on a half-micron scale, typically semiconductor devices, keen demand has arisen for development of devices, of higher density and higher degrees of integration. Therefore, more advanced photolithographic technology is needed for micro-pattern formation.
As one means to make patterns finer, there is a method in which an exposure light having a shorter wavelength is employed for the formation of resist patterns. At present, studies are energetically conducted on the use, as a light source for exposure, of a KrF excimer laser (wavelength=248 nm) instead of an i-line (wavelength=365 nm) in mass-production processes of 256 Mbit DRAMs (process dimension 0.25 .mu.m or less).
However, for the manufacture of DRAMs having an integration degree of 1 Gbit or more--which requires processing technology on more minute levels (process dimension 0.18 .mu.m or less)--a light source having an even shorter wavelength is needed. Specifically, photolithography by use of an ArF excimer laser (193 nm) has recently been proposed (Donald C. Hoffer, et al., Journal of Photopolymer Science and Technology, Vol. 9 (No. 3), p387-p397 (1996)).
Thus, the development of negative photoresists suitable for use in photolithography that employs an ArF excimer laser has been demanded. In relation to the development of such resists for use with an ArF excimer laser, a laser of improved cost performance is desired, in view that the source gas for laser oscillation has a short service life; a laser apparatus itself is expensive; and other factors. Therefore, in addition to a higher level of resolution that corresponds to a more minute process dimension, there arises a strong demand for higher sensitivity.
Chemically amplified resists making use of a photoacid generator--which is a photosensitizing agent--have been well known to improve sensitivity of resists. As a typical example, mention may be given of the invention disclosed in Japanese Patent Application Laid-Open (kokai) No. 2-27660. This publication discloses a resist containing triphenylsulfonium hexafluoroarsenate and poly(p-tert-butoxycarbonyloxy-.alpha.-methylstyrene) in combination. At present, such a chemically amplified resist is widely used as an excimer laser resist (e.g., Hiroshi Ito, C. Grantwilson, American Chemical Society Symposium Series Vol. 242, p11-p23 (1984)). Chemically amplified resists are characterized in that a photoacid generator serving as the resist component generates a protonic acid by light irradiation and the acid causes acid-catalyzed reaction with a resist resin, etc. through thermal treatment after exposure. Thus, sensitivity is dramatically enhanced compared with the case of a conventional resist having photoreaction efficiency (occurrence of reaction caused by one photon) of less than one.
At the present time most developed resists are of the chemically-amplified-type, and therefore the introduction of a chemical amplification mechanism is essential for the development of high-sensitivity materials that cope with shortening of the wavelength of an exposure light source.
However, in the case of lithography using light having a wavelength as short as 220 nm or less, typically an ArF excimer laser, a negative resist for forming micro-patterns must have new characteristics which conventional resist materials cannot possess, i.e., high transparency to exposure light having a wavelength of 220 nm or less and dry-etch resistance.
Conventional negative photoresists for g-line (438 nm), i-line (365 nm), and KrF excimer laser (248 nm) are mainly formed of a resin and a crosslinking agent, wherein a resin having an aromatic ring in the structural unit such as a novolak resin or poly(p-vinylphenol) is used as the resin component. Etch resistance of the resin is maintained due to the dry-etch resistance of the aromatic ring. With regard to the crosslinking agent, there are used an azide compound such as 2,6-di(4'-azidobenzal)-4-methylcyclohexanone or 3,3'-diazidophenyl sulfone, and a methylolmelamine resin. However, resins having an aromatic ring exhibit strong photoabsorption to light having a wavelength of 220 nm or less. For this reason, most exposure light is absorbed at the surface of the resist and the light cannot reach the substrate, so that fine resist patterns cannot be formed. Therefore, conventional resins cannot be adapted to photolithography employing light having a wavelength as short as 220 nm or less. Accordingly, strong need exists for negative photoresist materials having no aromatic ring, being endowed with etch resistance, and exhibiting transparency to light having a wavelength of 220 nm or less.
As polymer compounds having transparency to an ArF excimer laser (193 nm) and dry-etch resistance there have been proposed alicyclic polymers such as a copolymer having an adamantyl methacrylate unit (Takechi et al., Journal of Photopolymer Science and Technology, Vol. 5, No. 3, p439-p446 (1992)) or a copolymer having an isobornyl methacrylate unit (R. D. Allen et al., Journal of Photopolymer Science and Technology, Vol. 8, No. 4, p623-p636 (1995) and Vol. 9, No. 3, p465-p474 (1996)).
Previously, the present inventors have developed resins which serve as resist materials and also satisfy the above-mentioned requirements (Japanese Patent Application Laid-Open (kokai) Nos. 7-252324 and 8-259626). However, the resist materials disclosed therein are chemically amplified positive resists comprising a resin having an acid-decomposable group and a photoacid generator and provide a positive pattern. Thus, no negative resist satisfying the above requirements has yet been developed.
Accordingly, there exists need for a high-sensitivity negative resist having excellent transparency and etch resistance suitable for lithography employing an exposure light having a wavelength of 220 nm or less, particularly 180-220 nm, and for a pattern formation method using such a negative resist material. Moreover, there exists need for a method of manufacturing semiconductor devices including pattern formation making use of the pattern formation method of the present invention.